1. Field of the Invention
The field of the invention is the detection and control of the formation of adherent precipitates (such as scale, paraffin wax, etc.) on various surfaces.
2. Description of Prior Art
The formation of adherent precipitates on equipment surfaces immersed in liquids is a long-standing widespread and costly problem in industry. Such deposits reduce the rates of heat transfer, increase corrosion and erosion, clog flow lines, and interfere with the proper functioning of instruments and control systems.
The most common form of such troublesome coatings is adherent inorganic scale, which often precipitates from water used in industrial equipment. For example, insoluble deposits of alkaline earth metal carbonates and sulfates frequently precipitate on the surfaces of heat exchanger tubes, thus reducing by major amounts the rates of heat transfer. The fact that the tubes are hot is a primary reason for such scale formation.
Although adherent inorganic scale is the most common form of harmful precipitate, it is emphasized that adherent organic deposits are also major problems in certain industries. Thus, the formation of harmful precipitates is not confined to aqueous systems. For example, in the refining of oil, sticky adherent deposits form on metal surfaces of the reactors, heat exchangers, or transfer lines. These deposits are often the result of heating of the oil being processed, which heating changes or decomposes asphaltic constituents, asphaltenes or similar substances to form undesired adherent coatings. In other instances, cooling instead of heating is the cause of the problem. For example, crude petroleum oil will deposit adherent coatings of paraffin wax when the temperature of the oil, or of the surfaces over which it passes, is lowered sufficiently.
Scale or deposit formation is also a troublesome occurrence in many systems containing organic liquids. For example, deposits frequently occur in high wattage electrical transformers in which the windings are immersed in hydrocarbons or in halogenated aromatic compounds and the like; in hydraulic oil systems containing polyols, ethers and other organics; in heat-transfer liquid systems such as heavy oil, bisphenol A or similar highboiling organics; and in numerous organic chemical processing units.
Scale and other harmful coatings are likewise found in two-phase systems. For example, in the processing of freshly produced crude oil, the fluid is heated in a "heater-treater" unit to separate the unwanted salt water. Alkaline earth metal carbonates and sulfates are often present as adherent scale in such treating systems, the scale being sometimes mixed with various amounts of organic material.
There exists a major need for a practical, commercial method of determining whether or not a system is forming significant scale or other adherent precipitates, of determining the conditions under which scale might form, and of determining the conditions under which such formation can be prevented either by addition of chemical scale inhibitors or by control of process variables. It is highly important that the method be capable of implementation by commercial instruments, which function at all times and which do not require trained chemists or scientists for their operation. It is also extremely important that the method be so sensitive that the tendency of a system to develop scale will be detected without waiting until such formation has created substantial harm in the commercial system being monitored.
In the past, physical inspection of plant equipment has been the common method of ascertaining the presence and extent of adherent scale and other precipitates. Another common method has been to measure changes in heat transfer rates (or in required liquid flow velocities to maintain a certain heat transfer rate). Both of these common methods suffer from the fatal deficiency that the harm which it is desired to prevent (for example, lowered heat transfer rate) must occur before "preventive" measures can be taken. It is pointed out that, by the time scale and other deposits are visible, and by the time changes in heat transfer rates can be detected, the deposits are already so substantial as to create negative effects in the system. It is also to be understood that microscopic inspection of surfaces in actual industrial equipment is impractical and expensive, and that even macroscopic inspection is usually difficult, inconvenient and costly.
Because of the great difficulty of making physical inspections of the industrial equipment itself, one method of making heat exchanger studies is to specially design, construct and operate a laboratory model heat exchanger. Such a model usually includes windows for visual inspection, or includes means for withdrawing heat exchanger tubes so that they can be inspected and analyzed. Similarly, it is known to design laboratory heat exchangers wherein the heat transfer rates are monitored in relation to electrical power input, or steam condensation rates. Obviously, the construction and operation of such laboratory models is expensive and time-consuming and the data obtained with them may not be truly representative of what is occurring in the actual industrial equipment. Furthermore, reliance on changes in heat transfer rates, or on macroscopic inspection of surfaces, produces fatal insensitivity.
In addition to constructing and operating models of heat exchangers or other industrial equipment, there are frequently employed, in the laboratory, chemical methods related to formation of scale and similar substances. For example, test solutions are prepared which are basically unstable and will, in response to heating (or standing) and to the passage of time, yield precipitates of alkaline earth metal carbonates or sulfates. Different chemicals are added to such test solutions, and the degree to which such additives prevent or inhibit precipitation is determined. It is, however, emphasized that such tests do not provide continuous monitoring of an actual commercial system, nor do they necessarily produce significant data relative to formation of adherent scale in the actual system. It is to be noted that adherent scale or other precipitate is extremely harmful, but that those precipitates which are not adherent may be relatively harmless.
Other examples of laboratory procedures relative to scale, etc., involve determining the "stability" of the water in aqueous systems. Stability is ascertained by measuring or calculating from composition analysis, the minimum amount of acid or base required to effect precipitation. The amount of reagent tolerated by the solution without precipitation is taken as being proportional to stability and thus as being inversely proportional to the scale-forming tendency of the liquid. Such periodic tests can, at best, only be indirectly and uncertainly related to the tendency of an actual system to form adherent scale (or other) deposits.
Relative to prior-art patents in the present field, the following are exemplary: U.S. Pat. Nos. 2,931,219, 2,994,821, 3,080,747, 3,141,324 and 3,552,189. These patents teach methods which require visual inspection, weighing, heat transfer changes, or other slow and insensitive approaches. There are, in other and different fields, patents showing devices which make use of electrical contact resistance. These include U.S. Pat. Nos. 1,567,728, 2,107,604 and 3,411,082. The last-mentioned patents do not teach or suggest methods of detecting the onset of scale, etc., or of controlling scaling in an industrial system.
To summarize, therefore, all previous methods known do not detect or measure the first formation of adherent scale, etc., before such scale has caused significant harm, nor do they provide a way to test a particular liquid in order to learn in a relatively short time whether or not adherent scale will form under specified conditions. It is a major object of this invention to detect the onset of scaling or fouling, long before the deposit is either visible or causes a change in heat transfer rate, and without the necessity of removing the test surface from the liquid in which it is immersed. It is another major object to determine, quickly and easily, the conditions under which adherent scale, etc., will precipitate from various liquids.